With popularization of air bags and perfect duty to wear a seatbelt, the number of fatalities due to vehicle traffic accidents tends to decrease. However, because of increase in senior drivers due to aging, the number of traffic accidents and the number of injured persons still tend to be large. In view of such a background, for the purpose of assisting driving, attention is given to a sensor to detect any obstacle around a vehicle. So far, such sensors have been commercialized as ultrasonic sensors, cameras, milli-meter wave radars and the like.
A conventional vehicle-mounted radar can detect an obstacle that exists at a middle distance of less than 30 m or at a great distance of less than 150 m. However, for an obstacle at a short distance of less than 2 m, for example, its detection problematically has a large margin of error. In order to detect the obstacle near the vehicle precisely, there is a demand for the practical use of a UWB radar which has high axial resolution and ensures broader view.
The patent literature 1 (PL1) discloses an array antenna in which antenna elements are arranged in a 2×4 pattern. As an antenna element, disclosed is a printed antenna element formed by printing on a substrate. FIG. 30 illustrates an example of an array antenna formed by printing a plurality of printed antenna elements on the substrate together. FIG. 30A illustrates a linear array antenna 900a in which printed antenna elements 901 are arranged in a 1×4 pattern and FIG. 30B illustrates an array antenna 900b in which printed antenna elements 901 are arranged in a 2×4 pattern. Each printed antenna element 901 has one radiating element 902 and one second ground plane 903, which are printed on the substrate as one group. The Eθ component of the antenna element 901 is arranged in a vertical direction perpendicular to the radiation surface.
In these radars, a phase comparison monopulse system is used to measure a horizontal azimuthal angle of an object to detect around the vehicle. In the phase comparison monopulse system, reception signals received at two antennas arranged in the horizontal direction are used as a basis to obtain a value by normalizing a difference signal of both reception signals by a sum signal of the reception signals. Then, the value is applied to preset discrimination curve (monopulse curve) thereby to obtain a deviation angle in the vertical direction on the antenna plane.
Besides, the non-patent literature (NPL1) discloses an UWB radar antenna 910 as illustrated in FIG. 31. The antenna 910 is a linear antenna in which antennal elements 911 are arranged in a 1×4 pattern. Each antenna element 911 uses as a radiating element 912 a wide-coverage bowtie antenna by linear polarized wave, around which cavities 914 are provided with rims. In rims 915, through holes 916 electrically connected to a ground plane (not shown) are arranged at predetermined pitch.